Big Apple real-estate developers and landlords are raising the roof over a citywide crackdown on office tower roof terraces — which have recently become the must-have amenity for tenants.

Major new projects around town are imperiled or are being delayed by a bizarre, “out of left field” interpretation by the Department of Buildings of 1961 zoning language that was only intended to stop a proliferation of outdoor flea markets, multiple sources told The Post.

The 56-year-old zoning code stated that “all uses must be contained within enclosed buildings.”

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Although hundreds of office buildings around town have long had tenant-accessible terraces, DOB examiners are now interpreting that to mean that outdoor spaces — whether on building tops or on setbacks — can be used only for trees and plants, not by people.

Among many highly visible locations where new terraces are to be created, the Durst Organization plans to add roof and setback terraces to 1155 Sixth Ave. as part of a $110 million improvements program.

Durst spokesman Jordan Barowitz declined to talk specifics but said, “This is a problem. Access to outdoor space is a critical component of a healthy work environment and a vital part of sustainable development as well as a major recruitment tool for New York’s businesses.”

The DOB campaign, which is said to be pushed by First Deputy Commissioner Thomas Fariello, has delayed dozens of new commercial projects. The situation is so dire that the Real Estate Board of New York, a lobbying and advocacy organization, urged the Department of City Planning to “convince the DOB that what we’re asking is well within scope of what zoning allows,” REBNY’s Michael Slattery said.

One prominent real-estate executive, who didn’t want to be identified, raged about the DOB: “They take forever to approve anything, cranes collapse on their watch — but their priority now is to make up reasons not to allow terraces in office buildings.

“It makes no sense. [Mayor] de Blasio is pro-development. He rezoned Vanderbilt Avenue, where One Vanderbilt is going up with outdoor terraces — yet this agency is going rogue right under his nose,” the source said.

Insiders said the DOB harassment often takes the form of challenging whether a roof can support a certain amount of weight.

“It’s baloney,” one real-estate insider said. “There are open-air rooftop bars all over town that hold a zillion more people than an office terrace.”

Not only is DOB telling people their terrace plans may not be approved, but some owners who had already had theirs OK’d have been told they could have that approval rescinded, Slattery said.

Asked to comment, the DOB and the DCP gave The Post a murky, joint statement: “We are aware of the questions that have been raised regarding roof terraces and we are working together to arrive at a solution that supports both safety and clarity.”

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Rooftop outdoor spaces now command huge importance on the ferociously competitive office-leasing scene. Many tenants consider them to be as indispensable as open floor plans, on-site health clubs and digital infrastructure.